The present disclosure generally relates to direct conversion receivers used in telecommunication or data communication devices, and more specifically, to a local oscillator (xe2x80x9cLOxe2x80x9d) leakage signal canceller in such a receiver.
Wireless communication systems often require a frequency converter to translate the high frequency input signal to a lower frequency for further processing. This may be accomplished by mixing the input with a locally-generated frequency source. One approach to wireless reception may utilize direct conversion of the high frequency input signal to baseband. Direct converters translate the RF input directly to baseband with zero intermediate frequency. Since there are no intermediate frequencies, many filters may be omitted or simplified. Therefore, it may be desirable to use such direct converters in mobile cellular communications.
A common problem encountered in direct conversion receivers is the effective LO leakage isolation. Since frequency mixers have finite LO to RF isolation, some of the LO signal appears at the receiver input and may be observed at the antenna. If there is inadequate filtering, the magnitude of this re-radiated signal may exceed the limit set by regulatory agencies, such as Federal Communications Commission (FCC). Furthermore, the signal may have a detrimental effect on receiver performance since any LO leakage into the antenna may downconvert through the mixer to produce a spurious reception response. Such LO leakage is of a particular problem in direct conversion receivers because the leakage signal is at the same frequency as that of the input signal, and thus cannot be filtered.
In direct conversion receivers, the LO leakage can downconvert and contribute to a troublesome dc offset in the baseband section of the receiver. This offset appears in the middle of the downconverted signal spectrum, and may be larger than the signal itself and significantly larger than thermal and flicker noise. Thus, the LO leakage signal in the direct conversion receiver may operate to jam other nearby subscriber units. Furthermore, the LO leakage may cause unintended radiation to exceed FCC specifications.
The present disclosure describes a quadrature downconverter for a direct conversion receiver. The design of the downconverter is optimized to significantly reduce LO leakage signal leaking from the local oscillator through the RF section to the antenna.
The quadrature downconverter includes an RF input element, at least first and second quadrature splitters, a local oscillator, and at least two sets of mixers. The RF input element is configured to provide at least first and second RF inputs. The first and second quadrature splitters are configured to receive the first and second RF inputs and provide at least first and second in-phase RF outputs and at least first and second quadrature RF outputs.
The local oscillator operates to provide at least first and second reference signals. Each reference signal has an in-phase reference output and a quadrature reference output. Each set of mixers has at least first and second mixers. The first mixers are configured to modulate the first or second in-phase RF output with the in-phase reference output to produce in-phase baseband signals. The second mixers are configured to modulate the first or second quadrature RF output with the quadrature reference output to produce quadrature baseband signals. The quadrature splitters and the reference outputs operate to substantially reduce any local oscillator leakage signal leaking into the RF input element.